It's not that hard to build a RAID 5 NAS (see Build a Cheap and Fast RAID 5 NAS if you want to take a shot at it). The tough part is making one that doesn't take an MCSE or CCNA to set it up and keep it running. With the N5200, Thecus has clearly succeeded at making a RAID NAS that leads the (current) pack in performance. But, just as clearly, they have also produced a NAS with a user interface that only a geek could love and with plenty of work left to do.
GUI design and user-friendliness don't appear to have been at the top of Thecus' design goals for the N5200 and it shows in many ways. Documentation manages to cover the "what" and "where" for the most part, but is sorely lacking in the "why" and "how". There is no help available in the web interface, either, which is poorly organized and makes you hunt down important status information. As far as support, the Thecus website doesn't have a knowledge base, FAQ section or Forum. And while there is phone support from 10AM to 5PM Pacific time, you'll pay for the call.
Prospective buyers looking to use the 5200 as a media server will need to look elsewhere since the product includes no media servers. Thecus appears to be focused more on SMB than home use with the N5200, given its support for multiple networked file systems and Active Directory instead of UPnP AV and iTunes.
My bottom line is that if you're looking for the fastest NAS for file serving and backup and don't mind a funky user interface, sparse documentation, and immature firmware then you might take a chance on the 5200. But if it were my money, I'd go for an Infrant ReadyNAS NV (or NV+ if you want a front-panel display). The NV has a longer track record, mature firmware, much better user interface, more extensive feature set including multiple media servers and client backup, and is about $100 cheaper for a BYOD model. It's also more compact and much quieter—although it still won't be sitting within earshot of my media consumption space.
Thecus has succeeded in raising the feature bar for "prosumer" NASes to include five bays, RAID 6 and 10 and speeds that make good use of a gigabit LAN connection. But its reach has exceeded its grasp in producing an all-around polished product that anyone could feel comfortable buying.